When a personal telecommunications service system (sometimes called a cordless or wireless telephone service system) is provisioned at a customer site, it is made sure that enough base stations are provided to ensure that a single cordless (wireless) handset user can receive or make a call from any part of the building. In practice, traffic requirements are also considered and provisioning must take into account numerous wireless handset users who may be in relative proximity to the same base station. To send or receive a signal from a base station, a wireless handset must be within radio range of a base station. The radio range of a base station is determined by measuring the strength of the radio signal between it and a wireless handset, using an RDT (radio deployment tool). The RDT consists of an RDT base station containing a transmitter to send a test signal and an RDT wireless handset for measuring the strength of the test signal received from the RDT base station, called RSSI (received signal strength indication). A master control which connects by wire or wireless to control and record data concerning the operation of the base station and the RSSI sent back by the wireless handset may also be included.
Radio signals radiate from a base station uniformly in all directions when there are no obstructions to the signals. The same is true for receiving signals sent by a wireless handset. How far the radio signal travels depends on the construction materials of the walls, as well as the presence of other RF (radio frequency) barriers such as metalwork, concrete or marble walls, etc. which interfere with signal propagation. Since base stations (and wireless handsets) have a limited radio range, the building needs to be divided into units of coverage called cells. Base stations will be installed at the center of cells when provisioning is completed. Each cell is the area of radio coverage provided by one or more base stations in the same location to an acceptable RSSI level.
Provisioning of a building is conducted by partitioning the floor layouts into cells, as determined by the RDT. There is a specific threshold, called the hand-off threshold, used to measure a cell's range. This is to ensure continuous radio coverage as wireless handset users move throughout the building. The threshold has been set to allow the system to hand-off a call before the radio link is lost or the wireless handset is out of radio range. Specifically, hand-off is the capability of the system to transfer an active call from a radio in one cell to a radio in another cell.
Wireless handset users who are mobile require continuous radio coverage. Hand-off by the system allows calls to be transferred from a radio in one cell to a radio in another cell when the signal strength is becoming unacceptable. By way of example, presently the hand-off threshold is customarily set at -73 dBm. At this value, the radio signal is still adequate but the system will start looking for an available radio in another cell before the link is lost.
One of the main objectives of provisioning is to meet the customer's needs, using the least number of base stations. The strategy adopted for the wireless system requires the identification of the outer bounds of the coverage that the signal must reach. For a building, these are the outer walls. Partitioning the coverage area into cells begins at the extreme points of the building and continues towards the center. In this way, radio signals are kept within the building as much as possible. The outcome of provisioning should be the exact location of the minimum number of base stations necessary to meet the customer's needs.